The growing adoption of rich multimedia services has resulted in explosive demand for wireless data capacity. Recent studies indicate that wireless traffic has grown at a rate that is approximately an order of magnitude higher than spectral efficiency enhancements available to meet the required increase in capacity. This gap will increase further as the number of devices per person increases, and newer devices enable the consumption of even richer multimedia content.
Recent work in the area of heterogeneous networks (Het-Nets) has primarily focused on network overlay techniques for offloading data traffic to smaller cells. Heterogeneous networks overlay low power and low cost devices on coverage holes or capacity-demanding hotspots to supplement existing single-tier cellular networks. While large cells, covered by macro base stations (MBSs), for example, provide blanket coverage and seamless mobility, small cells served by devices like femto access points (FAPs), pico base stations (PBSs), WiFi access points (APs) and relay-stations (RSs) help provide coverage extension and boost local capacity. To boost capacity further, the network may leverage spectrum across different radio access technologies (RATs).
One of the key elements of long term evolution (LTE) is the need to coexist with other RATs. Deployments typically aim for full spectral reuse across the tiers and the cells in the network as licensed spectrum is expensive and scarce. The integrated small-cells used in multi-RAT heterogeneous network architectures may also co-locate WiFi®/LTE radio interfaces. Such integrated infrastructure lowers cost by leveraging common infrastructure across multiple RATs, such as common site acquisition, backhaul etc.
Interference is a concern for wireless communication due to the need to maximally reuse scarce spectral resources. In multi-tier networks, the interference problem is even more severe because there is additional cross-tier interference when the overlaid small cells reuse the same spectrum as macrocells. Typically, the interference can be mitigated via periodically muting the transmission of macro-cells (or small cells) to protect the transmission of small cells (macro-cells) at the cost of reducing overall spectral efficiency.
Given that an increasing number of clients in the network are equipped with multiple radio interfaces, e.g., WiFi® in addition to 4G, an operator can also exploit the different radio networks to add low-cost capacity, and improve coverage and quality of service (QoS) in the network. In addition to the multitier aspect of Het-Nets, multi-RAT network components contribute extra performance enhancement. In Multi-RAT Het-Nets, the availability of additional RATs (e.g. WiFi®) provides extra flexibility to partition the radio resources among users to mitigate the effect of interference.